The present invention relates to improvement of paper sheet feeders for use in image forming apparatuses such as copying machines, printers, and facsimile devices, and more particularly relates to an improvement of a separating arm holding mechanism in a sheet feeder utilizing a corner pawl separation method.
A sheet feeder for use in image forming apparatuses feeds paper sheets, sheet by sheet, by separating an uppermost sheet from a sheet stack housed in a feed tray. For such sheet feeding, as a sheet feeder for separating sheets constituting a sheet stack, sheet by sheet, there has conventionally been a sheet feeder of a separating arm method in which corner pawls are brought into contact with the top face at both end corner portions of each sheet on the downstream side of its feeding direction. The sheet feeder of this separating arm method is so structured that, as shown in FIG. 8, inside a rectangular casing tray 1 with an upper face opened, a separating arm 104 with a corner pawl 104a formed at its end portion is supported by an attachment plate 103 that is in contact with both side ends of a sheet stack 2 to regulate the sheet width.
A portion of the sheet stack 2 on the downstream side with respect to its feeding direction (hereinafter, referred to as a front portion of the sheet stack 2) is lifted from a bottom plate of the tray 1 by elasticity of an elastic member such as springs through an unshown lift plate, so that the top face of the uppermost sheet is normally in contact with the corner pawl 104a regardless of the volume of the sheet stack 2. Above the front portion of the sheet stack 2, an unshown semicircular feed roller is disposed so as to be rotatable. During feeding operation, the feed roller rotates so as to move a sheet to the downstream side of the feeding direction, while pressing down the entire sheet stack 2 against the elasticity of the elastic member to keep tight contact with the top face of the uppermost sheet of the sheet stack 2. In this operation, since the uppermost sheet of the sheet stack 2 is restrained from moving to the downstream side of its feeding direction by the corner pawl 104a, sagging occurs on the portion of the uppermost sheet on the downstream side in its feeding direction, as a consequence of which a resilience due to the sagging makes the sheet's both end corner portions on the downstream side in the feeding direction overpass the corner pawl 104a. This enables feeding of only the uppermost sheet of the sheet stack 2.
The separating arm 104 having the corner pawl 104a is supported by the attachment plate 103 so that the corner pawl 104a comes into contact with the top face of the uppermost sheet in the sheet stack 2 at an appropriate pressure in conformity with displacement of the top face of the uppermost sheet even when the entire sheet stack 2 is pushed down by rotation of the feed roller. More specifically, as shown in FIG. 9, the separating arm 104 having the corner pawl 104a is swingably supported at three points: a central swing-center support portion 105; an anterior swing guide portion 106; and a posterior swing guide portion 108, each formed on the attachment plate 103. These central swing-center support portion 105, the anterior swing guide portion 106 and the posterior swing guide portion 108 constitute a separating arm holding mechanism.
When the top face of the uppermost sheet is displaced downward by rotation of the feed roller, the rotation moment by its own weight is generated on the separating arm 104 around the central swing-center support portion 105. This rotation moment acts upon the top face of the uppermost sheet through the corner pawl 104a as a claw pressure, as a result of which the corner pawl 104a conforms to displacement of the top face of the uppermost sheet.
The swing range of the separating arm 104 may be such that it can cover an amount to which the sheet stack 2 is pushed down by the feed roller. Therefore, in consideration of easiness of manufacturing and assembling, the separating arm holding mechanism is extremely simple in structure. In consideration of a lifetime contact abrasion of a sheet feeder by paper sheets, the attachment plate 103 and the separating arm 104, which are made from metal plates, are formed each from one metal plate through stamping and bending by means of a press.
As for the central swing-center support portion 105, as shown in FIG. 12 and FIG. 13, a part of the attachment plate 103 is stamped into an inverted U shape, and the inverted U-shaped intermediate plate piece is bent at right angles to make a plate axis 109, in which arresting slits 109a symmetric with respect to the plate axis are punched. FIG. 13 is a cross sectional view taken on the line b—b of FIG. 12.
As shown in FIG. 14, in the separating arm 104, there is punched a central hole 112 with two insertion slits 112a being symmetrically cut therein. The insertion slits 112a are set to a width allowing the plate axis 109 of the central swing-center support portion 105 to pass through. In installing the separating arm 104 in the attachment plate 103, the separating arm 104 is rotated 90 degrees from the state shown in FIG. 14 so that the side on which the corner pawl 104a is formed is positioned on the lower side, and the plate axis 109 is inserted into the insertion slits 112a. Then, while being positioned inside the arresting slits 109a, the separating arm 104 is reversely rotated 90 degrees to restore a regular state. In FIG. 13, a two-dot chain line shows the separating arm 104 in the regular state. As shown in FIG. 14, the separating arm 104, which is supported by a narrow-width portion of the plate axis 109 of the attachment plate 103 at the central hole 112, is formed such that the core of the plate axis 109 is generally aligned with the center of the central hole 112. Thus, the separating arm 104 is engaged with the plate axis 109 of the attachment plate 103 at the central hole 112 so that the separating arm 104 is slidable and at the same time to arrest lateral movement of the separating arm 104 toward a direction perpendicular to a sliding axis.
In the anterior swing guide portion 106, as shown in FIG. 10, a cut slit 114 is provided in a bent portion 103a of the attachment plate 103. After the separating arm 104 is installed, swing of the separating arm 104 is guided by a full-face sliding portion 106a and an upper face sliding portion 106b. 
In the posterior swing guide portion 108, as shown in FIG. 9, a part of the attachment plate 103 is stamped into a horseshoe shape, and the horseshoe-shaped intermediate plate piece is bent in an L shape. On the top end surface thereof, there is formed a protrusion 108a as shown in FIG. 11. The separating arm 104 is brought into contact with the protrusion 108a during sliding so that only the outside of the lower rear portion (the side not facing the attachment plate 103) is guided to slide.
For installing the separating arm 104 in the attachment plate 103, the separating arm 104 is rotated 90 degrees to be engaged with the attachment plate 103, and then is put back to the original state as described before. In this operation, while a front portion of the separating arm 104 is inserted into the cut slit 114 provided on the anterior swing guide portion 106, a rear portion of the separating arm 104 is also engaged with the protrusion 108a of the posterior swing guide portion 108, by which the attachment plate 103 and the separating arm are warped against each other while being rotated. A stopper 113 shown in FIG. 10 is disposed in a position where the separating arm 104 can be inserted into the slit 114 of the attachment plate 103 in the state that the attachment plate 103 and the separating arm 104 are warped against each other, and where the separating arm 104 cannot slip out from the slit 114 once the separating arm 104 is inserted and the attachment plate 103 and the separating arm 104 are returned to the original state by elasticity.
The swingable range of the separating arm 104, which is determined by a length of the slit 114 and a position of the stopper 113, covers a pushed-down amount of the sheet stack 2 when the feed roller is rotated. Through such forming process, a structurally-stable separating arm holding mechanism by three-point support is made up from the minimum number of components without the necessity of any other components and tools in installation and attachment steps.
However, with the form of the anterior swing guide portion 106 shown in FIG. 10, if deflection occurs on the separating arm 104 and the attachment plate 103 in their manufacturing step, or if warpage generated in installing the separating arm 104 in the attachment plate 103 is so large that distortion remains, the sliding resistance in the full-face sliding portion 106a and the upper face sliding portion 106b during sliding operation increases, and therefore an effort of the separating arm 104 upon paper sheets drastically changes, which works against the separation performance for separating sheets of paper. As a result, there would occur paper feeding errors such as multiple sheet pages being fed, the corner of sheet being folded, and paper jam.
Also, when the separating arm 104 is installed in the attachment plate 103 as described above, in the central swing-center support portion 105 shown in FIG. 9, the upper and lower insertion slits 112a of the central hole 112 shown in FIG. 14 are rotated 90 degrees so as to be engaged with the plate axis 109 of the attachment plate 103 shown in FIG. 13, and then the separating arm 104 is reversely rotated 90 degrees by the arresting slits 109a of the plate axis 109 to restore a previous state. When the separating arm 104 is reversely rotated 90 degrees, it is necessary to align the center of the central hole 112 with the core of the plate axis 109. If they are not aligned, the plate axis 109 comes into contact with the insertion slits 112a, which disables rotation of the separating arm 104.
Accordingly in the conventional separating arm holding mechanism, for easy alignment of the core of the plate axis 109 and the center of the central hole 112, guides 110 are formed symmetrically on both sides of the plate axis 109 on the attachment plate 103 by stumping and bending process (see FIG. 12 and FIG. 13), whereas in symmetric positions around the central hole 112 on the separating arm 104, slits 111 in which the guides 110 are fit are formed in a circular-arc shape large enough to cover the swing range of the separating arm 104 (see FIG. 14). In installing operation, the separating arm 104 is rotated 90 degrees and is engaged with the plate axis 109 of the attachment plate 103 through the insertion slits 112a, by which the guides 110 are also inserted into the slits 111. When the separating arm 104 is reversely rotated 90 degrees, the guides 110 slide inside the slits 111, which facilitates alignment of the core of the plate axis 109 and the center of the central hole 112.
However, since gaps are present between the guides 110 and the slits 111, there has been a problem that even with use of the guides 110 and the slits 111, operations of aligning the core of the plate axis 109 with the center of the central hole 112 and rotating the separating arm 104 in a reverse direction are complicated.
Further, since a part of one slit 111 is positioned above the central hole 112 of the separating arm 104 (see FIG. 14), the central swing-center support portion 105 is positioned slightly lower than the corner pawl 104a of the separating arm 104. Consequently, a swing track of the corner pawl 104a is displaced backward at the time of upward movement, whereas at the time of downward movement, the swing track is displaced forward, which increases friction with paper sheets during swing operation, thereby bringing about a problem that an effect of the separating arm 104 upon paper sheets becomes instable. Further, since the central swing-center support portion 105 is positioned on the lower side, a distance between the central swing-center support portion 105 and the posterior swing guide portion 108 is short, which would pose a problem that the installed separating arm 104 suffers considerable lost motions, thereby causing degraded precision of a product.